1. Field of the Invention
The present invention relates to a magnetic sensor and position detector for use on a machine tool, industrial machine or the like to detect the position thereof.
This application claims the priority of the Japanese Patent Application No. 2003-033890 filed on Feb. 12, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Presently, a sensor utilizing the magnetoresistance of a film of a ferromagnetic substance such as Fe—Ni, Ni—Co or the like, that is, the so-called MR (magnetoresistance) sensor, is used as a magnetic sensor in many cases. The MR sensor is formed from a substrate of glass, silicon, ceramic or the like on which there is formed a ferromagnetic film (of about 50 to 100 nm in thickness) on which a pattern of leads and connecting terminals for providing signals to outside is formed from a low-resistance metallic material by the photolithography. It should be noted that the MR sensors include an AMR sensor utilizing the anisotropic magnetoresistance (AMR), GMR sensor formed from a magnetic layer and multilayer film of nonmagnetic conductor and utilizing the giant magnetoresistance (GMR), TMR sensor formed from a magnetic layer and multilayer film of an insulative material and utilizing the tunnel magnetoresistance (TMR), etc.
The MR sensor has a resistance which varies depending upon an external magnetic field, and it is used to provide a magnetic head to reproduce a magnetic signal recorded in a magnetic recording medium, a magnetic sensor used on a machine tool, industrial machine or the like to detect the magnitude and direction of an external magnetic field, etc.
In a machine tool or the like, the MR sensor is disposed opposite to a magnetic recording medium having alternating fields recorded therein at an equal pitch or at a constant wavelength or having some pole pattern to be movable relative to the magnetic recording medium. Therefore, the MR sensor can detect a periodic signal (generally sinusoidal-wave signal) by detecting a fringing field caused by a magnetic recording medium having the alternating fields, for example. In the machine tool or the like, the MR sensor detects a linear travel, rotational travel, relative position, etc. by counting the period of the periodic signal or a linear travel, rotational travel, relative position, etc. with a resolution smaller than a pitch at which alternating fields are recorded in a magnetic recording medium by electrically sub-dividing the periodic signal in a predetermined manner.
Also, since the fringing field caused by a magnetic recording medium having alternating fields recorded therein with a small resolution or at a small pitch (several tens μm to several mm) for detection of a position or angle is weak, the MR sensor has to be nearer to the magnetic recoding medium to such an extent that the fringing field can be detected. Generally, the distance between the MR sensor and magnetic recording medium has to be smaller than the recording pitch, namely, about a quarter to a half of the recording wavelength. Thus, the MR sensor should be placed to face at the surface thereof the magnetic recording medium in order to detect the weak fringing field.
In the construction in which the MR sensor surface is opposite to the magnetic recording medium, a mechanical vibration or the like will possibly cause the MR sensor and magnetic recording medium into contact with each other, which is likely to damage the MR sensor. Conventionally, an inorganic film of silicon oxide or silicone nitride is formed on the MR sensor for protecting the surface of the MR sensor, and an organic film of a polymer such as polyimide resin or epoxy resin is further formed on the inorganic film, to protect the surface of the MR sensor. It should be noted that the organic film protects the MR sensor primarily against mechanical vibration or the like and the inorganic film protects the MR sensor primarily against application of the stress of the organic film to the MR sensor. Practically, the inorganic film is of about 0.5 to 2 μm in thickness and the organic film is of about 2 to 8 μm in case it is formed from the polyimide resin or of about 20 μm in case it is formed from the epoxy resin.
Further, it has been proposed to use a carbon membrane having a diamond-crystalline structure (DLC film) as the protective layer (see the cited reference 1, for example).
Note that the epoxy resin can be formed into a film thicker (20 μm) than a one formed from the polyimide resin and the epoxy resin film can more effectively protect the MR sensor against any external shock or the like. So, it is more preferable to use the epoxy resin film as the protective layer. However, if the protective layer formed on the MR sensor is too thick, the MR sensor surface will be excessively apart from the magnetic recording medium. On this account, the magnetic recording medium should be a one causing a fringing field having an strength detectable by the MR sensor. Therefore, a magnetic recording medium having magnetic signals recorded therein with a short magnetic recording wavelength cannot be used because it causes only a weak fringing field.
For example, a magnetic recording medium having magnetic signals recorded therein with a short magnetic recording wavelength of about 80 μm should be about 20 to 30 μm from the MR sensor surface. Therefore, in case such a magnetic recording medium having magnetic signals recorded therein with such a short magnetic recording wavelength is used, the protective layer on the MR sensor should be formed thin. However, a thin protective layer can protect the MR sensor only against a relatively weak external shock. If a strong external shock is applied to the MR sensor, the protective layer will be broken and thus the MR sensor will be damaged.
Generally, the MR sensor will be damaged in many cases during assembling into a position detector, machine tool or the like. In some cases, a spacer formed from a mylar film to an appropriate thickness is placed between a magnetic recording medium and MR sensor to set an appropriate distance, a so-called clearance, from the magnetic recording medium during assembling of the MR sensor into a position detector or machine tool, as the case may be. In this case, however, the protective layer will possibly be broken and MR sensor will thus be damaged when the spacer is pulled out after the MR sensor is fixed. In some cases, a scratch arisen in the protective layer but not being any problem when the spacer is pulled out will possibly cause the MR sensor to fail later.
Also, while a machine tool is in operation, a foreign matter such as dust or saw dust enters the above-mentioned clearance and damages the protective layer and MR sensor in other cases. This is because even a foreign matter somewhat larger than the clearance breaks in the protective layer which is formed on the uppermost surface of the MR sensor from a material (high polymer) softer than the foreign matter.
The Japanese Published Unexamined Patent Application No. 2001-82978 cited herein (Cited Reference No. 1) discloses a DLC film as such a protective layer. This DLC film is well known for its extremely strong stress. Therefore, if the DLC film is formed as it is on the MR sensor surface, the MR sensor is applied with a stress which will influence the magnetoresistance coefficient (MR ratio) of the MR sensor. Especially, if the DLC film is formed on an MR sensor formed from a ferromagnetic film of Fe—Ni or Ni—Co by the method disclosed in the Cited Reference No. 1, the MR sensor will have the magnetoresistance coefficient (MR ratio) deteriorated and cannot perform to the full extent as a sensor.
Currently, the DLC film is generally formed by the ion plating, CVD method or the like. Carbon films each containing carbon as the base, produced by these methods, will have an amorphous structure. Therefore, the protective layer stated in the Cited Reference No. 1 (DLC film: carbon membrane having a diamond-crystalline structure) can hardly be formed by these methods.
Also, the HDD (hard disk drive) and the like use the MR sensor as a magnetic head. For protection against damage due to sliding on a disk, some of such magnetic heads have a DLC film formed on the surface thereof which is in contact with the disk. In the HDD, the DLC film is formed on the surface of the magnetic head which is in contact with a disk in the HDD, not directly on the MR sensor surface, as shown in FIG. 1. Therefore, in the HDD, the DLC film is formed on the magnetic head surface with no consideration to the stress of the DLC film. Also, the DLC films used in such applications has a thickness of several nm to several tens nm and thus they are of no use as a protective layer for an MR sensor intended for use as a sensor.